Routing Forwarding versus Routing –Forwarding: –to select an output port based on destination address and routing table –Routing: –process by which routing table is built

Routing •Forwarding table VS Routing table •Forwarding table •Used when a packet is being forwarded and so must contain enough information to accomplish the forwarding function •A row in the forwarding table contains the mapping from a network number to an outgoing interface and some MAC information, such as Ethernet Address of the next hop •Routing table •Built by the routing algorithm as a precursor to build the forwarding table •Generally contains mapping from network numbers to next hops

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Routing •Network as a Graph •The basic problem of routing is to find the lowest-cost path between any two nodes •Where the cost of a path equals the sum of the costs of all the edges that make up the path

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Routing •For a simple network, we can calculate all shortest paths and load them into some nonvolatile storage on each node. •Such a static approach has several shortcomings •It does not deal with node or link failures •It does not consider the addition of new nodes or links •It implies that edge costs cannot change •What is the solution? •Need a distributed and dynamic protocol •Two main classes of protocols •Distance Vector •Link State

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Distance Vector nEach node constructs a one dimensional array (a vector) containing the “distances”(costs) to all other nodes and distributes that vector to its immediate neighbors nStarting assumption is that each node knows the cost of the link to each of its directly connected neighbors

Distance Vector

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